Efficient amplifiers are important for RF applications. A typical RF device uses a portable power source, e.g., a battery. The operation of the RF device is enhanced when the demand on the battery is reduced. The RF circuits of a portable device, however, present one of biggest demands on the power source of a portable device. In particular, the amplifiers in RF circuits consume considerable power.
There are different methods of RF signal transmission. In some methods, the information to be transmitted is completely coded in the phase of the RF signals that are transmitted. GSM is an exemplary standard in which the information to be transmitted is completely coded in the phase of the RF signals. In other methods, at least some information is coded in the amplitude of the RF signals. In the latter case, it is important to attempt to advance conflicting goals in the design of the RF amplifier circuit. A first goal is high average efficiency, which makes better use of the available power from a power source. A second goal is high linearity, so that the amplifier does not distort the information carrying amplitude signal. There are a number of techniques and standards that use the RF amplitude for some or all of the information carried by the RF signals.
Modern wireless data transmission methods are intended to provide for high data rates, as the traffic carried on the RF signals includes voice and bit heavy data traffic, ranging from text messaging to image data, video data, and internet protocol data. The third generation (3G) wireless communication methods utilize spectrally efficient variable-envelope modulation schemes. One such scheme is the hybrid phase-shift keying (HPSK) scheme, which has been adopted for the wideband code division multiple access (WCDMA) standard. In WDCMA spectral re-growth due to the transmitter circuit distortion is strictly limited. This often translates to stringent and challenging linearity requirements for the radio frequency (RF) amplifiers, which constitute the end of the transmitter chain and are tasked to handle the highest signal levels.
Another important, although conflicting, design criteria is the amplifier power consumption. Since RF amplifiers consume a significant share of battery power in a portable device, their power efficiencies have a direct and determining impact on the time period of device operations before a recharge or replacement of the power source, e.g., a battery, is required. Efficiency should be maximized at the amplifier's peak power level without compromising the amplifier linearity. Additionally, however, efficiency should also be high during power back-off. Achieving efficiency during both conditions has proved difficult in practice. The WCDMA standard, for example, requires power control (attenuation) to be continuously and adaptively enforced to achieve equalization of signals received by a base station irrespective of the distance from the base stations of handsets within the coverage zone of the base station. Thus, an RF amplifier should exhibit high average efficiency to prolong battery life. The amplifier's bias should be adaptive. For small signal conditions, the quiescent current should be kept to its minimum to enhance the efficiency. For large signal conditions, the current should automatically rise such that high linearity is achieved.
Class AB (or B) bias is traditionally employed in RF amplifiers to provide an adaptive bias current. There are various types of amplifiers that achieve adaptive bias current. One type of amplifier is commonly referred to as inductor base bias feed amplifier. In this circuit, an inductor is coupled between the base of an output transistor and the output of a current bias circuit. A variation is the self base bias control circuit, which adds a current mirror for feedback in the current bias circuit to increase the current multiplication effect. The current mirror feedback variation (self base bias control) of the inductor base bias feed amplifier is discussed in Shinjo, et al, “Low Quiescent Current SiGe HBT Driver Amplifier Having Self Base Bias Control Circuit,” IEICE Trans. Electron., vol. E85-C, no. 7, pp. 1404-1411, July 2002.
Recognized problems with these and other amplifier circuits using inductors include the amount of real estate occupied by the inductors. Resistors, typically a small amount of polysilicon, take up far less space than inductors. A resistor base bias feed circuit omits inductors, but the backward impedance requirement of the circuit requires high value resistors. However, as the base current of the output transistor increases, so does the voltage drop across the resistor connected to the base of the output transistor. Any increase in the base current of the common-emitter amplifier thus causes a voltage drop at the base. The subsequent base-emitter voltage (Vbe) reduction at large signal conditions greatly limits the current boost. The higher the resistor value (good for the impedance requirement), the more the bias circuit resembles a constant-current bias (where the collector current could not rise as power input (Pin) increases).
A variation of the resistor base bias feed adds an additional current bias circuit to feed current into the base of the output transistor. The dual bias circuit with resistor base bias is discussed in Taniguchi et al. “A Dual Bias-Feed Circuit Design for SiGe HBT Low-Noise Linear amplifier,” IEEE Trans. Microwave Theory Tech., vol. 51, no. 2, pp. 414-421, February 2003.
To achieve linearity, resistor degeneration is used, but at the expense of amplifier gain. A resistor is connected to the emitter of the output transistor. In this circuit configuration, the emitter resistor raises the voltage at the emitter of the output transistor, thereby reducing the base-emitter voltage drop (VBE), and, accordingly, the current boost effects of the above-mentioned bias schemes. Inductor degeneration avoids raising the emitter voltage, but, as mentioned above, inductors present fabrication problems due to the amount of real estate required to accommodate inductors.